Resources of fossil origin, such as petroleum cuts, for the chemical industry will be exhausted some decades from now. That is why, for some years, industrial corporations have oriented their research and development activities toward so-called “biosourced” methods of synthesis using renewable natural raw materials.
Acrolein, an important synthesis intermediate for the chemical industry, is produced industrially by gas-phase oxidation of propylene by the oxygen of the air in the presence of catalytic systems based on mixed oxides. Its manufacture is thus dependent upon raw material of fossil origin.
Acrolein is a key intermediate for the synthesis of methylmercapto-propionaldehyde and methionine, a synthetic protein used as an animal feed supplement, which has been widely adopted as a substitute for fish meal. Acrolein is also used for producing acrylonitrile, glutaraldehyde or pyridine.
Acrolein is in particular the non-isolated synthesis intermediate of acrylic acid, which is a very important raw material, which can be used directly for obtaining a polymer of acrylic acid or, after esterification with alcohols, for producing a polymer of the corresponding ester. These polymers are used as they are or as copolymers in fields as varied as hygiene (for example in the production of superabsorbents), detergents, paints, varnishes, adhesives, paper, textiles, leather, etc.
Glycerol, obtained from vegetable or animal oils in the production of biodiesel fuels, is one of the raw materials envisaged as a substitute for propylene for producing acrolein, glycerol being submitted to a reaction of catalytic dehydration leading to acrolein. Such a method thus makes it possible to rectify the problem of replacement of propylene, and to respond to the concept of green chemistry in a more general context of protection of the environment.
The reaction of dehydration of glycerol to acrolein has already been the subject of numerous studies, notably in the search for catalytic systems ensuring complete conversion of glycerol, as well as a yield and selectivity for acrolein suitable for production on an industrial scale.
We may mention for example documents U.S. Pat. No. 5,387,720 and WO2006/087084, which describe a method of synthesis of acrolein in the presence of solid acid catalysts, characterized by their Hammett acidity; such catalysts may notably be selected from natural or synthetic siliceous materials, acidic zeolites, oxides impregnated with inorganic acids, mono-, di-, tri- or polyacids, oxides or mixed oxides or heteropolyacids.
Other catalytic systems with activity for dehydration of glycerol have been proposed, for example solids based on doped iron phosphate, partially salified heteropolyacids, heteropolyacids supported on porous titanium oxide, or mixed oxides of phosphorus and of vanadium (WO 2009/044081; WO 2009/128555; WO 2011/033689; WO 2010/046227).
However, the reaction of catalytic dehydration of glycerol to acrolein is always accompanied by secondary reactions leading to the formation of byproducts such as hydroxypropanone, propanaldehyde, acetaldehyde, and acetone, but also formation of high-boiling compounds such as phenol, polyaromatic compounds or compounds resulting from the polycondensation of glycerol.
The presence of byproducts requires steps of separation/purification for recovering either purified acrolein, or a stream containing acrolein that can be submitted to a subsequent step of oxidation for producing acrylic acid.
The high-boiling compounds are partly the cause of coke formation on the catalyst.
This leads to deactivation of the catalyst, and consequently a decrease in conversion of glycerol and in selectivity for acrolein.
The catalyst must therefore be changed periodically in order to maintain satisfactory economic efficiency. Catalyst life depends on the operating conditions of the system, and may range from a few hours to a few days. Periodical regeneration of the catalyst compensates the deactivation at least partially, giving satisfactory catalytic activity again, but greatly reduces plant productivity.
Many studies have thus related to improvement of productivity, notably by combining various operating conditions.
Thus, in document WO 2006/087083 in the name of the applicant, it is proposed to use molecular oxygen during the reaction of dehydration of glycerol to acrolein to reduce the formation of coke on the catalyst. In this method for producing acrolein, catalyst regeneration may be performed ex-situ, for example for a fluidized bed, by continuous extraction of spent catalyst and combustion under air, than reloading of fresh catalyst without stopping production. In this case, regeneration is carried out at a temperature and a pressure which are not necessarily the same as those for the reaction. Catalyst regeneration may also be carried out continuously in-situ at the same time as the reaction, taking into account the presence of a small amount of molecular oxygen in the reactor. In this case, regeneration is carried out at the temperature and at the pressure of the reaction, and is more like partial inhibition of deactivation: in fact, the oxygen content is not sufficient for conserving a sufficient activity of the catalyst after some tens of hours to some days.
In the two-step method for producing acrylic acid from glycerol described in document DE 10 2008038273, dehydration of glycerol is carried out in the presence of a catalyst divided into at least two parts, one part being in reaction mode and the other part in regeneration mode. The reaction is carried out at a temperature in the range from 200° C. to 400° C., whereas catalyst regeneration is carried out at a temperature in the range from 300° C. to 600° C., in oxidizing conditions using a gas containing oxygen, in particular a gas mixture comprising less than 10 vol % of oxygen, or in reducing conditions in the presence of a gas containing hydrogen. The two catalyst beds, fixed or fluidized, function alternately in reaction mode and in regeneration mode in two reactors in parallel.
In the method for producing acrolein described in document WO 2008/052993, the problem of regeneration of the dehydration catalyst is solved by employing a circulating bed reactor operating at a temperature between 200° C. and 650° C.; the catalyst, after separation of the reaction stream, is regenerated continuously in the presence of a gas containing oxygen at a temperature in the range from 400° C. to 700° C., with combustion of the coke supplying the heat that is used for the reaction, in particular for vaporizing the reactor feed stream.
Document US 2008/0214384 describes a method for producing acrolein from glycerol in the presence of a tungsten-based acid catalyst comprising at least one promoter selected from a list of elements, the presence of this promoter reducing the tendency for carbonization of the catalyst and facilitating its regeneration. The dehydration reaction is carried out at a temperature between 150° C. and 450° C. For regeneration carried out in the presence of oxygen or hydrogen, a high temperature is used, between 100° C. and 800° C., which does not correspond to the reaction temperature. The method may be implemented using reaction/regeneration cycles in two separate reactors to obtain a continuous stream of acrolein.
In patent application JP 2008-137950, it is proposed to use a dehydration catalyst containing a metal selected from Pt, Pd, Ru, Ir, Cu and Au, making it possible to prolong catalyst life and reduce the temperature and regeneration time, in a method for producing acrolein from glycerol.
In the method described in patent application JP 2008-110298, regeneration of the catalyst for glycerol dehydration is carried out at a temperature above the calcination temperature in order to reduce the regeneration time and return to a level of catalytic activity of the same order as the initial catalytic activity. The temperature of the catalyst during regeneration is controlled on the basis of the regeneration temperature, the concentration of oxidizing agent and the flow of oxidizing agent. A plurality of reactors operating either in reaction or in regeneration makes it possible to avoid interrupting the production of acrolein.
Document US 2011/0152582 describes particular plant configurations for carrying out a reaction of dehydration and for regenerating the phosphorus-based catalyst used. In this plant, feed of the reactive stream is stopped sequentially in order to feed the reactor with an oxidizing gas or a reducing gas for regenerating the catalyst.
Despite these various developments, there is still a need for new methods for further improving the productivity of the reaction of dehydration of glycerol to acrolein, and consequently the productivity of manufacture of acrylic acid from glycerol on an industrial scale.
Now, the inventors discovered that this need could be satisfied by combining cycles of reaction with cycles of regeneration in precise operating conditions, and these cycles can be integrated in a particular reactor configuration for continuous production of a stream containing acrolein.
One aim of the present invention is therefore to supply a method and a device for producing a stream comprising at least acrolein obtained by dehydration of glycerol, which can be fed continuously with glycerol efficiently in terms of energy consumption and productivity, without high capital costs and in good conditions of safety.